The first commercial resupply mission to the International Space Station is set to lift off from the Kennedy Space Center Sunday evening. From the space station crew's standpoint, some of the most precious cargo could well be ice cream.

The SpaceX Falcon 9 rocket with the Dragon capsule attached on a rollout demonstration test in Cape Canaveral, Fla. Tuesday. On Sunday night, the private space company will attempt to launch another capsule full of food, clothes and science experiments for the astronauts at the space station.

The mission, utilizing SpaceX's Falcon 9 rocket topped with the company's cargo-carrying Dragon capsule, follows on the heels of a successful test flight to the space station in May.

During that mission, Dragon delivered just over 1,000 pounds of cargo that NASA officials said wouldn't represent a significant set-back for the space-station program if something went wrong during the mission. This time, Dragon is carrying 882 pounds (nearly a ton when packaging is included) of more-precious cargo: experiments and hardware for the US, European, and Japanese laboratories; additional components needed to maintain the station; and crew supplies.

From the space station crew's standpoint, some of the most precious of all the cargo could well be ice cream Dragon is bringing up – not the freeze-dried kind, but real ice cream, kept in a new lab freezer Dragon will deliver. It's part of a shipment of "bonus food" the space agency periodically sends. The freezer is designed to preserve samples from biology and life-science experiments running on the station for return to Earth.

Indeed, Dragon's ability to return cargo to Earth is unique among the unmanned cargo craft the station's international partners provide. NASA's space shuttles were the only other vehicles able to do this. But NASA flew its final shuttle mission in July 2011, and the orbiters now are museum pieces.

All of the other cargo craft operating, as well as the capsule a second US company is building to supply commercial cargo service for NASA, become trash incinerators once they leave the station. They and the refuse they carry burn up on reentry.

Dragon is slated to return to Earth with 1,673 pounds of cargo, including components from the station's life-support system that failed and were replaced with on-orbit spares.

"It's good to be able to have that capability back," says Michael Suffredini, NASA’s space-station program manager. It saves money when hardware needs to be repaired and sent back to the station, and it allows engineers to examine the components to figure out why they failed.

And, he added during a pre-launch briefing, it's nice to have a US-based carrier back in the mix. It gives the US more flexibility in getting replacement parts to the space station in a timely fashion. Moreover, "shipping and customs can kill you when you're trying to get [parts] overseas" to ride on Russian, Japanese, or European cargo craft.

"We're excited to have the vehicle coming up to visit the ISS," he says.

The launch is set for 8:35 p.m. Eastern Daylight Time Sunday, although the weather forecast gives the mission a 40 percent chance of a scrub because conditions could violate launch requirements. Once off the pad, Dragon should take about 53 hours to reach the station, said Gwynne Shotwell, SpaceX's president. The capsule is expected to remain hitched to the space station for about three weeks.

Because Dragon is trying to rendezvous with another spacecraft, it has a very narrow span of time in which it can launch each day. The mission can't wait an hour for the weather to clear if it's not acceptable at launch time. If tonight's launch is postponed, SpaceX will have opportunities Monday and Tuesday for launches, days when the weather is expected to be more favorable for a launch.

In addition to the weather, NASA is eying some space debris that may require the space station to move out of the way. The debris is expected to make its closest approach to the station on Monday at 7:02 a.m. Eastern Daylight Time. Any change in the station's orbit would be slight, Mr. Suffredini said. But it would require Dragon to work a bit harder to catch up with the station.

As SpaceX was making its final preparations for today's launch last week, Orbital Sciences Corporation – the second company slated to carry cargo to the space station – cleared a milestone enroute to its first test launch.

On Oct. 1, it rolled out the first stage of its Antares rocket to the launch pad at NASA's Wallops Island Flight Facility on the Virginia coast. By the end of October, NASA officials say, they anticipate the first fuel loading and unloading tests, as well as the first, brief test of the rocket's motors as the first stage stands on the pad. In December, the company plans its first test flight, followed by the first demonstration flight to the space station.

NASA and Orbital Science haven't decided on timing for the demonstration flight, Suffredini says. "But I'd be surprised if it was before late February or early March."